NRAO Home > CASA > CASA Toolkit Reference Manual
image.rotate - Function

1.1.1 rotate the direction coordinate axes attached to the image and regrid the image to the rotated Coordinate System


Description

This function rotates two axes of an image. These axes are either those associated with a Direction coordinate or with a Linear coordinate. The Direction coordinate takes precedence. If rotating a Linear coordinate, it must hold precisely two axes.

The method is that the Coordinate is rotated and then the input image is regridded to the rotated Coordinate System.

If the image brightness units are Jy/pixel then the output is scaled to conserve flux (roughly; just one scale factor at the reference pixel is computed).

A variety of interpolation schemes are provided (you need only specify the first three characters to method). The cubic interpolation is substantially slower than linear. By default you get cubic interpolation.

You can specify the shape of the output image (shape). However, all axis that are not regrided retain the same output shape as the input image shape for that axis. Only the direction coordinate axes are regridded.

You can also specify a region-of-interest to be applied to the input image. If you do this, you need to be careful with the output shape for non-regridded axes (must match that of the region - use function boundingbox to find that out).

If outfile is given, the image is written to the specified disk file. If outfile is unset, the on-the-fly Image tool returned by this function is associated with a temporary image. This temporary image may be in memory or on disk, depending on its size. When you destroy the on-the-fly Image tool (with the done function) this temporary image is deleted.

The argument replicate can be used to simply replicate pixels rather than regridding them. Normally (replicate=F), for every output pixel, its world coordinate is computed and the corresponding input pixel found (then a little interpolation grid is generated). If you set replicate=T, then what happens is that for every output axis, a vector of regularly sampled input pixels is generated (based on the ratio of the output and input axis shapes). So this just means the pixels get replicated (by whatever interpolation scheme you use) rather than regridded in world coordinate space. This process is much faster, but its not a true world coordinate based regrid.

As decribed above, when replicate is False, a coordinate is computed for each output pixel; this is an expensive operation. The argument decimate allows you to decimate the computation of that coordinate grid to a sparse grid, which is then filled in via fast interpolation. The default for decimate is 0 (no decimation). The number of pixels per axis in the sparse grid is the number of output pixels for that axis divided by the decimation factor. A factor of 10 does pretty well. You may find that for very non-linear coordinate systems (e.g. very close to the pole) that you have to reduce the decimation factor.

The output pixel mask will be good (T) unless the regridding failed to find a value for that output pixel in which case it will be bad (F). For example, if the total input mask (default input pixel mask plus OTF mask) for all of the relevant input pixels were masked bad then the output pixel would be masked bad (F).

Arguments





Inputs

outfile

Output image file name. Default is unset.

allowed:

string

Default:

shape

Shape of output image. Default is shape of input image.

allowed:

intArray

Default:

-1

pa

Angle by which to rotate. Default is no rotation.

allowed:

any

Default:

variant 0deg

region

Region selection. See ”help par.region” for details. Default is to use the full image.

allowed:

any

Default:

variant

mask

Mask to use. See help par.mask. Default is none.

allowed:

any

Default:

variant

method

The interpolation method. String from ’nearest’, ’linear’, or ’cubic’.

allowed:

string

Default:

cubic

decimate

Decimation factor for coordinate grid computation

allowed:

int

Default:

0

replicate

Replicate image rather than regrid?

allowed:

bool

Default:

false

dropdeg

Drop degenerate axes

allowed:

bool

Default:

false

overwrite

Overwrite (unprompted) pre-existing output file?

allowed:

bool

Default:

false

stretch

Stretch the mask if necessary and possible? See help par.stretch. Default False

allowed:

bool

Default:

false

Returns
image

Example

 
 
"""  
ia.maketestimage()  
imr=ia.rotate(outfile="rotated.im", pa=’45deg’)  
imr.done()  
ia.close()  
 
 
In this example, we rotate the direction coordinate axes (RA/Dec) of a  
test image by 45 degress and regrid the image onto the axes.  

__________________________________________________________________


More information about CASA may be found at the CASA web page

Copyright 2016 Associated Universities Inc., Washington, D.C.

This code is available under the terms of the GNU General Public Lincense


Home | Contact Us | Directories | Site Map | Help | Privacy Policy | Search